The present invention is directed toward the field of Electromagnetic Interference ("EMI") shielding, and more particularly to a system for shielding discrete areas on printed circuit boards in order to minimize the transmission of EMI from one area to another.
Many electronic devices, such as communication equipment, use electrical circuits and components that generate and/or use high-frequency time-varying voltage signals. A naturally occurring byproduct of these high frequency signals is electromagnetic ("E/M") fields. The E/M fields can interfere with and affect the functioning of adjacent electrical components which are sensitive to EMI. In addition, federal regulations exist that limit the amount of E/M energy an electronic device can emit. Therefore, it is desirable in the design of electrical devices to provide shielding to reduce the level of E/M energy that particular circuits and components emit.
In many electronic devices, electrical components are mounted on printed circuit boards ("PCBs") and connected to other components mounted on the same PCB. Components of different circuits may be mounted on the same PCB. The E/M fields created by the components of the various circuits on the PCB may interfere with components of other circuits on the same or on a nearby PCB. As a result, it is desirable to place one or more shields on a PCB to surround the components that generate substantial EMI in order to protect other components from the E/M fields.
The miniaturization of electronic devices presents additional challenges when designing circuits that do not interfere, electromagnetically, with each other. Because less space is available in laying out miniaturized circuits, EMI-generating components may be placed in close proximity to EMI-sensitive components on the same PCB. Also, electronic devices having multiple PCBs may the PCBs arranged such that the components of one PCB face the components of another PCB. This could result in EMI-sensitive components being placed in close proximity to EMI generating components from other PCBs within the same electronic device.
Present methods for providing EMI shielding on PCBs include at least four techniques. In one method, a die cast frame is soldered to the PCB. The walls of the frame divide the PCB into discrete areas which are to be shielded. A single lid is used to cover the entire frame including all of the discrete areas. A compressible conductive material is applied to the inside of the lid in order to insure that the lid maintains mechanical and electrical contact with the interior walls of the frame. The lid is fastened to the frame using dimples that snap over a ledge on the outside section of the frame wall. This method of shielding has many deficiencies. First, there is only one lid that encloses the multiple discrete areas. As a result, E/M energy can be transmitted between adjacent areas. EMI generated in one discrete area can be transmitted to adjacent discrete areas of the PCB by conduction through the lid. Second, because one lid is used to enclose the entire circuit board, the lid has to be tall enough to enclose the tallest electronic component on the PCB. As a result, this method of shielding does not allow designers to minimize space through the technique of nesting PCBs whereby electronic components are mounted on PCBs such that a tall component from one PCB will face a short component from a facing PCB. With no nesting, the spacing between facing PCBs will have to be at least as great as the height of the tallest component on the PCB plus the width of the shield plus some minimum clearance distance.
In another method, a metallic or conductive coated plastic implement, which comprises both the walls and lid of the shield, is used to shield discrete sections of a PCB. The shield is connected to the solder track of the PCB through the use of metal spring fingers or a conductive elastomer gasket attached to the shield's casting. This method suffers from several disadvantages. First, for this method to effectively shield EMI, both the PCB and the casting must be highly stiff and flat to ensure continuous contact between the casting, gasket and PCB. Second, this method requires extra board space because bolting or some other type of mechanical retention system is needed to fasten the casting to the circuit board with sufficient force to provide contact between the solder track on the PCB and the gasket on the casting. Third, this method makes rework of a PCB more difficult. The entire shield must be removed if rework of the PCB is needed.
In a third method, formed sheet metal boxes are soldered directly onto a track on the PCB. The shields are, therefore, permanently attached to the PCB. If the shield has to be removed for rework of the circuit board, the shield must be desoldered which often damages or destroys the PCB.
In a fourth method, sheet metal walls or fences are provided which can be surface mount soldered onto a circuit board. A formed sheet metal lid is snapped or pressed onto the walls to provide an RF seal. This method also suffers from several disadvantages. First, this method does not provide a high level of shielding between adjacent areas on the PCB because a single lid is used to enclose all of the shielded areas. Second, space between PCBs cannot be minimized because the lid will have to be tall enough to enclose the tallest electronic component on the PCB and effective nesting of components could not be achieved.